Silver halide photographic materials form images by development processing after imagewise exposure. In black-and-white photographic materials such as direct medical X-ray films, black-and-white films for general photographing, litho films, scanner films or black-and-white photographic papers, silver halide grains in exposed areas are reduced by reducing agents such as hydroquinones, phenidones or aminophenols to form a silver image. In color photographic materials based on color development process an oxidation reduction reaction occurs between silver halide in exposed areas and color developers exemplified by p-phenylenediamines and the oxidized color developers couple with photographic couplers to form a color image. In color diffusion transfer photographic materials, a method of forming a positive image is known wherein direct positive silver halide emulsion designed to be developable only in non-image areas is associated with diffusible dye releasers (DRR compounds) releasing diffusible dyes upon reaction between the oxidized DRR and hydroxy anions.
The above mentioned silver halide black-and-white and color photographic materials have an important problem in common, that is, to restrain fog in non-image areas or, in other words, to improve discrimination between an image and background fog.
The term "discrimination" as used herein is intended to mean the distinction between fog in non-image areas and in image areas. This term is explained in the following literature:
(1) P. J. Hillson, "Discrimination and Development--The Influence of Excess Energy of Small Development Centers on the Kinetics of Development", Journal of Photographic Science, Vol. 22, page 31 (1974). PA0 (2) P. J. Hillson, "Discrimination in Photographic Development", Photographic Science and Engineering, Vol. 23, page 40 (1979).
Latent image nuclei are formed by exposing negative silver halide emulsions to light. The subsequent development renders the latent image nuclei available as starting points of the development to form silver images as well as imagewise distribution of oxidized developers. Color development forms a dye image upon the coupling reaction between the oxidized developer and image forming couplers. Silver halide grains in non-exposed or non-image areas should not be reactive at all during development but are in practice partially developed and, therefore, fog is produced due to undesirably developed silver in black-and-white development while color stain as well as the above mentioned silver fog are generated in color development. Such undesirable phenomena are speculated to occur by various mechanisms: silver halide grains have developable fog nuclei already at the emulsion preparation stage; fog nuclei are formed by the influence of heat, humidity, pressure or harmful gas during storage of photographic films or papers; fog nuclei are generated by developing agents during development. Such fog formation due to the above mechanisms becomes a barrier to increasing photographic speed of modern silver halide photographic materials. In particular, p-phenylenediamine derivatives as primary color developers have the defect that they are apt to cause fog in spite of their comparatively low reactivity among developers and this becomes a restriction to accelerating development processing or to increasing photographic speed.
Therefore, the development of techniques to remove or to decrease fog is an important subject in the art, and the use of various antifoggants has been proposed. Examples of effective antifoggants include benzimidazoles having a nitro group, indazoles, nitrogen-containing heterocyclic compounds having a mercapto group (e.g., mercaptotetrazoles, mercaptooxazoles, mercaptothiazoles, mercaptotriazoles or benzotriazoles as disclosed in T. H. James, The Theory of the Photographic Process, 4th Ed., Macmillan (1977), pp. 396-399).
Further, low molecular weight antifoggants and polymer antifoggants are described in U.S. Pat. Nos. 3,157,509, 3,295,976, 3,342,596, 3,536,489, 3,576,638, 3,597,199, 3,598,599, 3,598,600 and 3,741,765, Japanese Patent Publication Nos. 6147/71, 19039/71, 4417/72, 10692/74, 41056/76 and 27933/78 and Japanese Patent Application (OPI) Nos. 18539/72, 43923/75 and 59463/80 (the term "OPI" as used herein refers to a "published unexamined Japanese patent application"). However, these antifoggants are unsatisfactory because the development in image areas as well as fog in non-image areas is restrained to such an extent that the image density greatly decreases, or because the sensitivity of the silver halide emulsion is lowered due to absorption of an antifoggant to silver halide grains at the exposure stage. Recently, U.S. Pat. Nos. 3,649,267, 3,888,677, 4,307,175, 4,310,612 and 4,350,752, Japanese Patent Publication Nos. 39727/79, 9696/80, 76541/82, 136640/82, 1140/83, 93442/84 and 105640/84 disclose a method of preventing sensitivity decrease of silver halide emulsions in which an antifoggant precursor is added to a photographic material and an antifoggant is rendered utilizable during development. The addition of such an antifoggant precursor seems to solve one of the problems caused by direct addition of an antifoggant but leaves the deterioration of discrimination unsolved owing to decrease of image density which arises from the restrained development in image areas. The greatest problem of the aforementioned prior art is caused by the fact that an antifoggant is present uniformly not only in non-image areas but also in image areas and thereby restrains fog generation in non-image areas and necessary image development as well. This problem has been substantial.